Subroutines Just like C, PERL offers the ability to use subroutines for all the same reasons – Code that you will use over and over again – Breaking large programs into a series of smaller, more manageable pieces – Making programs more readable – Making programs easier to test – Allowing for multiple programmers to work on a single program

Subroutines Recall from C – Subroutines have a name – Subroutines have an argument/parameter list for input values – Subroutines have a return type for output value(s) – Subroutine variables have local (to the subroutine) scope PERL is no different in these respects except for the last item – scope

Scope in PERL When a variable is used without declaring it, by definition it has global scope – That is, it can be used anywhere within the program $str = ‘abcde’; $regexp = ‘^ab*a$’; – Both $str and $regexp are globally scoped variables – If their contents is changed from within a subroutine, it will also change outside of the subroutine This is not how C works

Scope in PERL But, this “feature” of PERL can be turned off though the use of a special keyword/construct $str = ‘abc’; print $str, “\n”; can be changed to my($str) = ‘abc’; print $str, “\n”; to make the variable $str locally scoped to the block in which it is being used

Scope in PERL Enforcing scope and variable declaration – Also recall that in C we absolutely must declare all variables prior to usage – PERL has no such restriction Causes problems if you misspell variable names Causes problems if you use the same variable name more than once – We can make PERL act like C by including the following directive (it’s not a PERL code statement) at the top of our code file use strict;

Scope in PERL This program will not compile: use strict; $str = ‘abc’; print $str, “\n”; But this one will use strict; my($str) = ‘abc’; print $str, “\n”; Note that there is still no data typing! We’re just declaring the usage of a variable name

Back to subroutines Subroutine format sub mysubroutine { my($arg) my($localvar); # do something here return $localvar; }

Argument passing All of the previous should be fairly clear except for the line my($arg) This is how arguments are passed in – Note that there is no argument list in the subroutine definition – Arguments get lumped together as a special array called _ (underscore) – The line above pulls the arguments out of the array and places them into locally scoped scalar variables (just one in this example)

Argument passing More than one argument my($arg1, $arg2) Arguments are passed by value – Even if you alter their value within the subroutine, their value on the outside remains unchanged Passing arrays gets complicated because they all get lumped into a single array – This can be avoided by passing by reference but… – The syntax is horrible and I’m not going to burden you with it during the last week of class

Calling a subroutine Just do the obvious… $arg = ‘abc’; $retvar = mysubroutine($arg); print $retvar, “\n”; Or, better yet my($arg) = ‘abc’; my($retvar) = mysubroutine($arg); print $retvar, “\n”;

Practice Write a subroutine that takes one argument and returns it with the string “ modified by subroutine” appended – Recall the append is done with the. operator $var = $var.‘append me’; Write a subroutine that takes two arguments, one string and one regular expression, and returns 1 if they match, 0 if the do not – Read the string and the regular expression from the keyboard (don’t forget to chomp them before sending to the subroutine)

And more practice Convert your code to print out a file 30 lines at a time to use a subroutine – That is, make a subroutine that is called like this: printfile($filename); – In your main program, use a while loop to read a filename from the keyboard, then print the file – Halt the main program (while loop) when the user types a carriage-return with no filename